Single use syringe having safety shield

ABSTRACT

A single use syringe assembly includes a barrel having a chamber for retaining fluid, an open proximal end and a distal end including an elongate tip extending distally therefrom having a passageway therethrough in fluid communication with the chamber. A needle cannula having a lumen therethrough is connected to the elongate tip so that the lumen is in fluid communication with the passageway. An elongate needle shield having a longitudinal opening therein is hingedly connected to the barrel. A needle shield is capable of pivoting from an open position or a closed needle protecting position. The plunger including an elongate body portion and a stopper is slidably positioned in fluid-tight engagement with the inside surface of the barrel. Structure is provided for locking the plunger in the barrel by applying distally directed force to the plunger after fluid has been delivered from the chamber.

BACKGROUND OF THE INVENTION

The present invention relates generally to single use syringes havingsafety features and more specifically to a single use syringe having aplunger locking mechanism and safety shield.

In the United States and throughout the world, the multiple use ofhypodermic syringe products that are intended for single use only, isinstrumental in drug abuse and more particularly in the transfer ofdiseases. Intravenous drug users who routinely share and reuse syringesare a high-risk group with respect to the bloodborne pathogens includingHIV and AIDS. Also the effects of multiple use are a major concern inunder-developed countries where repeated use of syringe products may beresponsible for the spread of many diseases. Reuse of single usehypodermic syringe assemblies is instrumental in the spread of drugabuse even in the absence of infection or disease.

Many attempts have been made to remedy this problem. Some of theseattempts have required a specific act to destroy the syringe after useeither by using a destructive device or providing a syringe assemblywith frangible zones so that the syringe could be rendered inoperable byapplication of force. Other attempts have involved the inclusion ofstructure which would allow the destruction or defeating of the syringefunction through a conscious act by the syringe user. Although many ofthese devices work quite well, they do require the specific intent ofthe user followed by the actual act to destroy or render the syringeinoperable.

Furthermore, in the medical arts, a syringe assembly typically includesa sharp pointed needle for administering fluids to patients eitherdirectly or into intravenous apparatus, and in various blood drawingapplications either with syringes or with specialized holders forfilling evacuated tubes. Since needles are widely used for medicalprocedures, many people can be exposed to needles in the routine courseof their work.

Accidental needle sticks from used hypodermic needles can transmitdisease. Accordingly, various types of needle shields such as hingedneedle shields, have been designed to reduce the possibility ofaccidental sticks.

Various means have been provided for locking a hinged needle shield inthe closed (needle protecting) position. Needles are available in anumber of gauges and lengths so that they can be used for differentpurposes. Where a needle shield having a deflectable locking member isused to entrap a needle, it is important that the needle displace thelocking member or members as it enters the needle shield cavity. It isalso important that, since the needle is entrapped by the deflectablelocking member or members, it cannot easily be displaced from thecavity. A relatively large diameter needle can more easily displace adeflectable locking member than a small diameter needle, both enteringthe needle shield cavity and exiting the cavity. As a needle shieldshould preferably be usable to protect needles of various sizes, thedeflectable locking member or members should be designed such that it issufficiently flexible to allow even a relatively small diameter needleto deflect it as it enters the needle shield cavity, but providessufficient resistance to prevent the needle from being re-exposedthrough the opening of the cavity.

SUMMARY OF THE INVENTION

The present invention is directed to a single use syringe havingmultiple safety features. These safety features include a plungerlocking mechanism, which when activated, locks the plunger in the barrelso that the syringe may not be reused. The plunger locking mechanismincludes a locking ring on the plunger and a locking projection near theproximal portion of the barrel. Alternatively, the plunger lockingmechanism may include a pair of locking rings or discs. The plungerlocking mechanism further includes either a flexible portion on theplunger or a compressible stopper. When an additional axial load orforce is applied to the proximal end of the plunger, the flexibleportion of the plunger flexes allowing the locking ring to axialdisplace itself beyond the locking projection, thereby locking theplunger within the barrel. Alternatively, if a compressible stopper isused, when an additional axial force is applied to the proximal end ofthe plunger, the stopper compresses, allowing the locking ring toaxially displace itself beyond the locking projection of the barrel,thereby locking the plunger in the barrel. The flexible plunger and thecompressible stopper may be used together. Alternatively, a syringeassembly may include only one of the flexible plunger or thecompressible stopper.

Another safety feature of the present invention is a breakable proximalportion of the plunger. After the plunger has been locked within thebarrel, if an attempt is made to withdraw the plunger from the barrel,the proximal portion will detach or break off from the plunger to ensurethat the syringe is not reused.

Yet another safety feature of the present invention is a hinged shieldfor shielding the needle cannula after an injection

The present invention is advantageous because each of the safetyfeatures may be activated using a single hand. Another advantage of thepresent invention, is that conventional procedures for performing aninjection may be utilized.

A single use syringe of the present invention comprises a barrelincluding a cylindrical side wall having an inside surface defining achamber for retaining fluid. The barrel includes an open proximal endand a distal end having a distal wall with an elongate tip extendingdistally therefrom having a passageway therethrough in fluidcommunication with the chamber. A needle cannula having a proximal end,a distal end and a lumen therethrough is connected to the elongate tipso that the lumen is in fluid communication with the passageway of thetip. The connection of the needle cannula to the tip can be eitherdirectly or indirectly through a hub attached to the needle cannula. Anelongate needle shield is hingedly connected to the barrel. The needleshield includes two sidewalls defining a longitudinal opening and a backwall between the sidewalls defining a recess having an interior surface.The needle shield is capable of pivoting from an open position whereinthe needle cannula is exposed, to a closed needle protecting positionwherein the distal end of the needle cannula is within the longitudinalopening of the shield. A plunger including an elongate body portionhaving a proximal portion and a distal portion and a stopper is slidablypositioned in fluid-tight engagement with the inside surface of thebarrel for drawing fluid into and driving fluid out of the chamber bymovement of the stopper relative to the barrel. The elongate bodyportion extends outwardly from the open proximal end of the barrel. Thesyringe assembly further includes means for locking the plunger in thebarrel by applying an additional distally directed force to the plungerafter fluid has been delivered from the chamber.

The syringe assembly may also include plunger weakening structure forallowing the plunger to break upon application of excessive forceintended to move the plunger proximally after the plunger has beenlocked in the barrel. This structure may take the form of a reducedcross-sectional thickness in the proximal portion of the elongate bodyportion of the plunger.

Means for locking the plunger to the barrel may include a contractibleportion on the plunger, a discontinuity on the plunger and adiscontinuity on the barrel. The plunger discontinuity is capable ofengaging the barrel discontinuity when an additional distally directedforce which is applied to the plunger shortens the contractible portionso that the plunger discontinuity moves distally to engage the barreldiscontinuity to lock the plunger in the barrel. The barreldiscontinuity may be a recess or a projection and is preferably aninwardly directed projection in the shape of an annular ring. Likewise,discontinuity on the plunger may be a recess or a projection. Thediscontinuity is preferably an outwardly directed projection in theshape of an annular ring.

The contractible portion of the plunger may include one or more flexibleelements traversing a gap in the elongate body portion. The one or moreelements are capable of withstanding the forces of fluid delivery anddeflectable upon application of an additional force to the plunger. Thecontractible portion may also include a cavity formed by the distal endof the plunger and the interior surface of the stopper wherein theplunger moves into the cavity upon application of the additional forceto effectively shorten the length of the plunger.

The syringe assembly may further include structure for locking theneedle shield in the closed needle protecting position when the needleshield is pivoted into the closed position. The needle shield lockingstructure may include an arm projecting from the interior surface of theneedle shield. The arm includes a free end positioned so that when theneedle shield is pivoted to the closed position, the needle cannulamoves past the free end and is trapped in the needle shield by the arm.The needle shield locking structure may also include locking members onthe proximal end of the needle shield capable of engaging a cooperatingledge at the distal end of the barrel when the needle shield is pivotedto the closed position.

The syringe assembly may further include a needle assembly comprisingthe needle cannula and a hub having an open proximal end containing acavity and a distal end attached to the proximal end of the cannula sothat the lumen is in fluid communication with the cavity of the hub. Theneedle assembly is removably attached to the tip of the barrel byengagement of the tip in the cavity of the hub so that a lumen is influid communication with the chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of the syringe assembly accordingto the present invention.

FIG. 1A is a perspective view of the syringe assembly according to thepresent invention.

FIG. 2 is a cross-sectional side elevation view of the syringe assemblyof the present invention, with the plunger in position to perform aninjection.

FIG. 3 is a cross-sectional side elevation view of the syringe assemblyof FIG. 2, with the plunger in position after an injection has beenperformed.

FIG. 3A is an enlarged cross-sectional side elevation view of the distalend of the syringe assembly of FIG. 3.

FIG. 4 is a side elevation view of the plunger of FIG. 2.

FIG. 5 is a perspective view of the plunger of FIG. 2.

FIG. 5A is an enlarged perspective view of the contractible portion ofthe plunger of FIG. 5.

FIG. 6 is a perspective view of a plunger according to anotherembodiment of the present invention.

FIG. 6A is an enlarged perspective view of the contractible portion ofthe plunger of FIG. 6.

FIG. 6B is an enlarged perspective view of the distal portion of theplunger of FIG. 6.

FIG. 6C is a partial cross-sectional side elevation view of the syringeassembly of FIG. 6 with the plunger in position to perform an injection.

FIG. 7 is a partial perspective view of the proximal portion of theplunger according to the present invention.

FIG. 8 is an enlarged perspective view of the distal end of a plungeraccording to still another embodiment of the invention.

FIG. 9 is a cross sectional side elevation view of the safety shield ofFIG. 1.

FIG. 9A is a bottom plan view of the safety shield of FIG. 1

FIG. 9B is a side elevation view of the safety shield of FIG. 1.

FIG. 10 is a bottom perspective view of the safety shield of FIG. 1 inits closed position.

FIG. 11 is a partial cross sectional view of the syringe assembly ofFIG. 2, after the plunger locking mechanism has been activated.

FIG. 12 is a perspective view of the syringe assembly according to thepresent invention, after the proximal portion of the plunger has beenbroken off.

FIG. 13 is a partial cross sectional side elevation view of the syringeassembly of FIG. 6, after the plunger locking mechanism has beenactivated.

DETAILED DESCRIPTION

Referring to FIGS. 1-5A, a syringe assembly 20 according to the presentinvention, generally comprises a barrel 22, a plunger 24, a needleassembly 26 and a safety shield assembly 28.

Barrel 22 includes a proximal end 30, a distal end 32, an inside surface34, a fluid chamber 36 and an elongate tip 38 extending from distal end32 having a passageway 40 therethrough in fluid communication withchamber 36. The barrel further includes a collar 42 around elongate tip38 having a thread 43 on an inside surface 44 of the collar. Thisconfiguration is often referred to as a locking luer collar. The presentinvention contemplates the utilization of any type of syringe barrel andneedle or needle assembly assembly, and not just a syringe having alocking luer collar. Thus, embodiments of the present invention mayinclude an integral needle and syringe barrel, a luer slip tip with orwithout a collar, or any other needle/syringe configuration. Adiscontinuity such as locking projection 45 extends inwardly from theinside surface 34 of the barrel at its proximal end 30. As shown, thelocking projection 45 is an annular ring. It is within the purview ofthe present invention for the discontinuity to have various shapes andconfigurations, including locking barbs, as long as the discontinuitylocks the plunger in the barrel upon activation, as will be discussedbelow.

Syringe assembly 20 of the present embodiment preferably includes aneedle assembly 26 having a cannula 11 having a proximal end 12, adistal end 13 and a lumen 14 therethrough. The needle assembly furtherincludes a hub 15 having an open proximal end 16 having a cavity 17therein, and a distal end 18 joined to proximal end 12 of cannula 13 sothat lumen 14 is in fluid communication with cavity 17. The hub includesoutwardly extending projections 19 and is placed on the distal end ofbarrel 22 by aligning the distal end of the barrel with the cavity inthe hub so that the outward projections 19 of the hub engage thread 43in the collar. Needle assembly 26 is then rotated or screwed into thelocking luer collar 42 so that the needle assembly is held tightly onthe distal end 32 of the syringe barrel 22 through interaction of thelocking luer collar thread 43 and the projections 19 on the needle huband a frictional interference fit between elongate tip 38 on the barreland cavity 17 in the hub. Thus, the lumen 14 is in fluid communicationwith fluid chamber 36. It is within the purview of the present inventionto include a needle assembly having one-piece construction wherein thecannula and the hub are formed of one piece and a cannula attacheddirectly to the barrel without the use of a hub.

Plunger 24 includes an elongate body portion 46 having a proximalportion 48, a distal portion 50 and a stopper 52 disposed on the end ofdistal portion 50. Elongate body 46 includes a plurality of outwardlyprojecting axial ribs 54. Preferably, the plunger includes four ribs,but may include fewer or more than four ribs. According to the presentinvention, the plunger defines contractible portion 56. The termcontractible portion as used herein shall include structure which isflexible, collapsible, breakable and/or deformable or the like.Preferably contractible portion 56 is located along the distal portionof the plunger, but may be located anywhere along the plunger. In thisembodiment, the contractible portion comprises flexing elements 57disposed on each of the ribs. Each of the flexing elements in thisembodiment, comprises a weakened portion of the rib that flexes or bendsinwardly. Contractible portion 56 of the plunger is capable ofwithstanding the axial load of a typical injection. Application of anaxial force greater than that required for a typical injection willresult in the contractible portion of the plunger flexing deforming,breaking and/or collapsing, as will be discussed in greater detailbelow. Thus, the flexing elements 57 of this embodiment are strongenough to withstand the axial load of injection during normal use of asyringe.

Plunger 24 further includes a discontinuity such as annular locking ring58 that is preferably located on proximal portion 48. In thisembodiment, the locking ring is preferably chamfered such that itsdiameter decreases in the distal direction. On the distal side of thelocking ring 58, each rib preferably includes a slot 60. On the proximalside of the locking ring, each rib is configured to define a break point62. At its proximal end, the plunger includes a thumb press 64.

Stopper 52 includes an inside surface 66 and an outside surface 68. Thestopper 52 fits over distal portion 50 of plunger 24 at an annulardistal end 70 of the plunger. Inside surface 66 of the stopper includesan annular groove 72 for receiving the annular distal end 70 of theplunger. Thus, the stopper fits onto the plunger in a snap fitarrangement. Friction may also play a role in fitting the stopper ontothe distal end of the plunger. The stopper further defines a cavity 74.The outside surface 68 of the stopper includes at least one seal rib 76for sealing engagement with inside surface 34 of barrel 22. The plungerfurther includes a distal shoulder 78 abutting the proximal end ofstopper 52.

FIG. 6, 6A and 6B show an alternate embodiment of the invention wherelike parts are similarly numbered. A plunger 124 includes a contractibleportion 156 comprising a pair of platforms 159, each preferably havingtwo pairs of flexing columns 157 extending therefrom and attaching to aflexing platform 161. The pairs of columns 157 are offset from eachother preferably by 180 degrees. Flexing portion 156 is capable ofwithstanding the axial load of a typical injection. Application of anaxial force greater than that required for a typical injection willresult in the contractible portion 156 of the plunger flexing, as willbe discussed in greater detail below.

FIG. 8 shows yet another embodiment of a plunger 224 according to thepresent invention where like parts are similarly numbered. Plunger 224includes a contractible portion 256 comprising comprises two crossingflex members 257 connecting a pair of platforms 259. Contractibleportion 256 is capable of withstanding the axial load of a typicalinjection. Application of an axial force greater than that required fora typical injection will result in contractible portion 256 of theplunger flexing collapsing, breaking and/or deforming, as will bediscussed in greater detail below. It is within the purview of thepresent invention to include plunger rods having different geometries,shapes and materials for the contractible portion.

FIGS. 9, 9A, 9B and 10 show the safety shield 28. The safety shieldcomprises a hub 80 for engagement with collar 42 of barrel 22. Hub 80includes mating geometry on its inside surface that cooperates with anannular ring 82 on collar 42 of the barrel. The mating geometry may be,for example an annular ring or a pair of projections extending from theinside surface of hub 80. Thus, the shield attaches onto the collar viathe hub in a snap fit arrangement. It is within the purview of thisinvention to provide for mating of the hub and collar other than a snapfit, such as an interference fit, threads, adhesive, welding,deformation and the like.

The hub further includes edge 83 extending from its proximal end. Shield28 further comprises a hinge that preferably includes a living hingesuch as hinge 84. Hinge 84 connects the hub 80 to a shield member 86.Shield member 86 includes proximal portion 88 and distal portion 90. Theproximal portion 88 includes locking members 92. Locking members 92cooperate and latch onto ledge 83 when the safety shield is activated bymoving it to the needle protecting position. Proximal portion 88 is wideenough to receive needle hub 15, when the safety shield is activated.

Distal portion 90 of the shield member receives and retains the needlecannula 11 when the safety shield is activated, that is, pivoted to itsneedle protecting position. Needle cannula 11 is secured within thedistal portion 90 of the shield member 86 by retaining members or arms94 having free ends 95, preferably located on the interior surface ofshield member 86. Retaining members 94 allow for the needle cannula topass therebetween while the shield member is rotated into activation,and are angled so that the needle is locked behind the retaining membersonce shield member 86 is fully activated and locked by locking members92.

According to the present invention, the syringe assembly includesseveral features that can be used separately or in combination to ensurethat syringe is used only a single time. Referring now to all of thefigures, especially FIGS. 3 and 11, the operation of the syringeassembly is described. In operation, the syringe can be filled from avial, ampoule or other suitable container using know safe procedures.The syringe is filled by inserting the needle cannula into a vial andwithdrawing the plunger 24. This will cause the liquid to be drawnthrough the lumen of the needle cannula and into fluid chamber 36. Animportant advantage of the present invention is that the plunger can bemoved back and forth along the barrel as many times as necessary toproperly fill the fluid chamber. For example, the syringe barrel may befilled with sterile water and the sterile water can be injected into avial contain lyophilized medication which is then drawn back into thesyringe barrel. Contractible portion 56 of the plunger is strong enoughto withstand the axial forces necessary for filling the syringe. Manysingle use syringes in the prior art only allow one proximal motion ofthe plunger with respect to the barrel. With these prior art single usesyringes, once the plunger is moved in a distal direction with respectto the barrel, it can no longer be withdrawn. Therefore, mixing sterilewater and a lyophilized medication as described above is not possible.

Fluid 37 in fluid chamber 36 can now be injected into a patient ordelivered in another suitable manner such as through the pierceableseptum of a catheter connector. This occurs by applying an axial forceto the thumb press 64 to cause plunger 24 to advance within barrel 22,thereby expelling the fluid through the lumen of needle cannula 11. Theposition of the plunger at the end of the injection is shown in FIG. 3.

Upon completion of the injection, an additional axial force indicated asC in FIG. 3 is applied to the proximal portion of the plunger,preferably on thumb press 64. FIG. 11 shows the plunger in the lockedposition after the additional axial force C has been applied to theplunger. The additional axial force causes the flexing portions 57 toflex inwardly. As the flexing portions flex, the additional axial forcefurther causes locking ring 58 on the plunger to advance distally in thebarrel past locking projection 45, in the barrel, as shown in FIG. 11.Thus, the plunger is locked in the barrel and the syringe is disabledand unusable.

In addition to or as an alternative to the action of the flexingportions, the additional axial force can cause the stopper 52 tocompress to allow for distal axial movement of the locking ring 58 intothe locked position. The compression of the stopper 52 allows the distalend 70 of the plunger to axially advance within the chamber 74 of thestopper 52. This, in turn, allows for axial travel of locking ring 58 inbarrel 22 to a position beyond locking projection 45. The plunger 24,therefore, is locked in the barrel via the locking ring being advancebeyond locking projection 45 of the barrel. Thus, the syringe isdisabled and rendered unusable.

An alternate embodiment of the invention may include a support pin onthe distal end 70 of the plunger. The support pin would support thestopper during an injection. In addition, at the end of the injection,the support pin may pierce or puncture the stopper, if the compressionof the stopper does not provide enough axial travel for the locking ringto lock.

The present invention contemplates the use of contractible portion 56 ofthe plunger and the compressible stopper 52 either in combination orseparately to achieve the axial travel required to lock the locking ring58 in position beyond the locking projection 45. Thus, a plunger havinga contractible portion 56 according to the present invention may be usedin a two-piece syringe assembly, where there is no stopper and theplunger and the stopper are integrally formed, usually of the samematerial.

After locking ring 58 is locked within the barrel via annular lockingprojection 45, the plunger can no longer be withdrawn from the barrel.Any attempt to withdraw the plunger from the barrel will result in theproximal portion of the plunger breaking off. Application of an axialforce to the plunger in the proximal direction will cause the proximalportion of the plunger to break off at breaking points 62, since theplunger is locked within the barrel, as shown in FIG. 12.

The chamfer on locking ring 58 provides the locking ring with a onedirectional stiffness feature. The chamfer provides more flexibility tothe plunger locking ring while the plunger is axially advanced withinthe barrel 22, but less flexibility of the locking ring while theplunger is axially withdrawn from the barrel. Slots 60 located on thedistal side of locking ring 58 assist in achieving the one directionalflexibility of the locking ring by acting as a flex point during axialadvancement into the barrel. Since the slots have a minimal gap, theslots 60 only allow a minimal deflection when the plunger is beingwithdrawn from the barrel. In addition, the same one directionalstiffness may be achieved through the presence of two thinner annularrings in place of the singe locking ring 58 and slots 60.

As discussed, the contractible portion has enough stiffness to resistpremature collapse during injection, but provide enough compliance ofthe plunger rod during activation to achieve the axial movement requiredto reach the final locked position. Of course, the axial force requiredfor an injection varies based on the gauge and size of the needle andviscosity of fluid being injected.

FIG. 13 shows the plunger of the other embodiment of the inventionillustrated in FIG. 6, 6A and 6B in the locked configuration. In thisembodiment, application of an additional axial force C to the proximalend of the plunger causes flexing platform 161 to flex allowing fordistal axial movement of locking ring 58 into barrel 22 of the syringe.Locking ring 58 is locked in position via locking projection 45 on thebarrel 22. Thus, plunger 124 is locked into the barrel and the syringeis rendered disabled and unusable. After the plunger is locked,application of an axial force in the proximal direction will cause theproximal end of the plunger to break away, completely disabling thesyringe, as shown in FIG. 12.

According to the embodiment of the invention shown in FIG. 8, theadditional force applied to the proximal end of the plunger will resultin the flexing cross members 257 compressing together so that platforms259 move toward each other to allow for axial movement of the lockingring 58, thereby locking the plunger within the barrel and rendering thesyringe unusable.

After the syringe has been disabled, the safety shield 26 may beactivated to shield the needle cannula and protect against accidentalneedle sticks. The safety shield is activated by application a force inthe direction of arrow D in FIG. 12 to the proximal portion 88 of theshield. This will activate the living hinge, causing the shield member86 to rotate to the position shown in FIGS. 11 and 13. Needle cannula 11of the needle assembly is received between retaining members 94, andlocking members 92 cooperate with the edge 83 to lock the shield inposition.

One advantage of the present invention is the disabling features andsafety shield may be activated with a single hand, further protectingthe user.

The present invention contemplates the use of several safety features,either alone or in combination. The safety features include a flexingplunger with a locking ring to lock the plunger within the barrel, acompressible stopper with a locking ring to lock the plunger within thebarrel, a plunger having a proximal portion which breaks off, and aneedle safety shield.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention.

1. A single use syringe assembly comprising: a barrel including acylindrical side wall having an inside surface defining a chamber forretaining fluid, an open proximal end and a distal end including adistal wall with an elongate tip extending distally therefrom having apassageway therethrough in fluid communication with said chamber; aneedle cannula having a proximal end, a distal end and a lumentherethrough, said proximal end of said cannula being connected to saidelongate tip so that said lumen is in fluid communication with saidpassageway; an elongate needle shield hingedly connected to said barrel,said needle shield having two side walls defining a longitudinal openingand a back wall between said side walls defining a recess having aninterior surface, said needle shield capable of pivoting from an openposition wherein said needle cannula is exposed, to a closed needleprotecting position wherein said distal end of said needle cannula iswithin said longitudinal opening of said shield; a plunger including anelongate body portion having a proximal portion, a distal portion and astopper slidably positioned in fluid-tight engagement with said insidesurface of said barrel for drawing fluid into and driving fluid out ofsaid chamber by movement of said stopper relative to said barrel, saidelongate body portion extending outwardly from said open proximal end ofsaid barrel; and means for locking said plunger in said barrel whenapplying an additional distally directed force to said plunger afterfluid has been delivered from said chamber.
 2. The syringe assembly ofclaim 1 further including plunger weakening means for allowing saidplunger to break upon application of excessive force intended to movesaid plunger proximally after said plunger has been locked to saidbarrel.
 3. The syringe assembly of claim 1 wherein said means forlocking includes a contractible portion on said plunger, a discontinuityon said plunger, a discontinuity on said barrel, said plungerdiscontinuity capable of engaging said barrel discontinuity when anadditional distally directed force is applied to said plunger shortenssaid contractible portion so that said plunger discontinuity movesdistally to engage said barrel discontinuity to lock said plunger insaid barrel.
 4. The syringe assembly of claim 3 wherein saidcontractible portion of said plunger comprises one or more flexibleelements traversing a gap in said elongate body portion, said one ormore elements capable of withstanding the forces of fluid delivery anddeflectable upon application of said additional force.
 5. The syringeassembly of claim 3 wherein said contractible portion on said plungerincludes a cavity formed by a distal end of said plunger and an interiorsurface of said stopper, said plunger moving into said cavity uponapplication of said additional force.
 6. The syringe assembly of claim 3wherein said discontinuity on said barrel includes an inwardly directedprojection on said inside surface of said barrel.
 7. The syringeassembly of claim 6 wherein said inwardly directed projection is anannular ring.
 8. The syringe assembly of claim 3 wherein saiddiscontinuity on said plunger is an outwardly directed projection. 9.The syringe assembly of claim 8 wherein said outwardly directedprojection is an annular ring.
 10. The syringe assembly of claim 1further including means for locking said needle shield in said closedneedle protecting position when said needle shield is pivoted into saidclosed position.
 11. The syringe assembly of claim 10 wherein saidlocking means includes an arm projecting from said interior surface ofsaid needle shield, said arm having a free end positioned so that whensaid needle shield is pivoted to said closed position, said needlecannula moves past said free end and is trapped in said needle shield bysaid arm.
 12. The syringe assembly of claim 10 wherein said lockingmeans includes locking members on a proximal end of said needle shieldengaging cooperating ledge at said distal end of said barrel when saidneedle shield is pivoted to said closed position.
 13. The syringeassembly of claim 2 wherein said weakening means includes an area ofreduced cross-sectional thickness in said proximal portion of saidelongate body portion of said plunger.
 14. The syringe assembly of claim1 further comprising a needle assembly including said cannula and a hubhaving an open proximal end containing a cavity and a distal endattached to said proximal end of said cannula so that said lumen is influid communication with said cavity, said needle assembly beingremovably attached to said tip of said barrel through engagement of saidtip to said cavity so that said lumen is in fluid communication withsaid chamber.
 15. The syringe assembly of claim 1 wherein said stopperis made of material selected form the list consisting of thermoplasticelastomers, natural rubber, synthetic rubber, thermoplastic materialsand combinations thereof.
 16. A single use syringe assembly comprising:a barrel including a cylindrical sidewall having an inside surfacedefining a chamber for retaining fluid, an open proximal end and adistal end including a distal wall with an elongate tip extendingdistally therefrom having a passageway therethrough in fluidcommunication with said chamber; a needle assembly including a needlecannula having a proximal end, a distal end and a lumen therethrough anda hub, said hub having an open proximal end containing a cavity and adistal end attached to said proximal end of said cannula so that saidlumen is in fluid communication with said cavity, said needle assemblybeing removably attached to said tip of said barrel through engagementof said tip to said cavity so that said lumen is in fluid communicationwith said chamber; an elongate needle shield hingedly connected to saidbarrel, said needle shield having two sidewalls defining a longitudinalopening and a back wall between said sidewalls defining a recess havingan interior surface, aid needle shield capable of pivoting from an openposition wherein said needle cannula is exposed, to a closed needleprotecting position wherein said distal end of said needle cannula iswithin said longitudinal opening of said shield; a plunger including anelongate body portion having a proximal portion, a distal portion and astopper slidably positioned in fluid-tight engagement with said insidesurface of said barrel for drawing fluid into and driving fluid out ofsaid chamber by movement of said stopper relative to said barrel, saidelongate body portion extending outwardly from said open proximal end ofsaid barrel; and means for locking said plunger in said barrel whenapplying an additional distally directed force to said plunger afterfluid has been delivered from said chamber, said means for lockingincluding a contractible portion on said plunger, a discontinuity onsaid plunger, a discontinuity on said barrel, said plunger discontinuitycapable of engaging said barrel discontinuity when an additionaldistally directed force is applied to said plunger shortens saidcontractible portion so that said plunger discontinuity moves distallyto engage said barrel discontinuity to lock said plunger in said barrel,said contractible portion of said plunger comprising one or moreflexible elements traversing a gap in said elongate body portion, saidone or more elements capable of withstanding the forces of fluiddelivery and deflectable upon application of said additional force. 17.The syringe assembly of claim 16 further including plunger weakeningmeans for allowing said plunger to break upon application of excessiveforce intended to move said plunger proximally after said plunger hasbeen locked to said barrel.
 18. A single use syringe assemblycomprising: a barrel including a cylindrical sidewall having an insidesurface defining a chamber for retaining fluid, an open proximal end anda distal end including a distal wall with an elongate tip extendingdistally therefrom having a passageway therethrough in fluidcommunication with said chamber; a needle cannula having a proximal end,a distal end and a lumen therethrough, said proximal end of said cannulabeing connected to said elongate tip so that said lumen is in fluidcommunication with said passageway; an elongate needle shield hingedlyconnected to said barrel, said needle shield having two sidewallsdefining a longitudinal opening and a back wall between said side wallsdefining a recess having an interior surface, said needle shield capableof pivoting from an open position wherein said needle cannula isexposed, to a closed needle protecting position wherein said distal endof said needle cannula is within said longitudinal opening of saidshield; a plunger including an elongate body portion having a proximalportion, a distal portion and a stopper slidably positioned influid-tight engagement with said inside surface of said barrel fordrawing fluid into and driving fluid out of said chamber by movement ofsaid stopper relative to said barrel, said elongate body portionextending outwardly from said open proximal end of said barrel; andmeans for locking said plunger in said barrel when applying anadditional distally directed force to said plunger after fluid has beendelivered from said chamber, said means for locking including acontractible portion on said plunger, a discontinuity on said plunger, adiscontinuity on said barrel, said plunger discontinuity capable ofengaging said barrel discontinuity when an additional distally directedforce is applied to said plunger shorts said contractible portion sothat said plunger discontinuity moves distally to engage said barreldiscontinuity to lock said plunger in said barrel.
 19. The syringeassembly of claim 18 further including plunger weakening means forallowing said plunger to break upon application of excessive forceintended to move said plunger proximally after said plunger has beenlocked to said barrel.
 20. The syringe assembly of claim 18 wherein saidcontractible portion of said plunger comprises one or more flexibleelements traversing a gap in said elongate body portion, said one ormore elements capable of withstanding the forces of fluid delivery anddeflectable upon application of said additional force.
 21. The syringeassembly of claim 18 wherein said contractible portion on said plungerincludes a cavity formed by a distal end of said plunger and an interiorsurface of said stopper, said plunger moving into said cavity uponapplication of said additional force.
 22. The syringe assembly of claim18 further including means for locking said needle shield in said closedneedle protecting position when said needle shield is pivoted into saidclosed position.
 23. The syringe assembly of claim 22 wherein saidlocking means includes an arm projecting from said interior surface ofsaid needle shield, said arm having a free end positioned so that whensaid needle shield is pivoted to said closed position, said needlecannula moves past said free end and is trapped in said needle shield bysaid arm.
 24. The syringe assembly of claim 22 wherein said lockingmeans includes locking member son a proximal end of said needle shieldengaging cooperating ledge at said distal end of said barrel when saidneedle shield is pivoted to said closed position.
 25. The syringeassembly of claim 18 further comprising a needle assembly including saidcannula and a hub having an open proximal end containing a cavity and adistal end attached to said proximal end of said cannula so that saidlumen is in fluid communication with said cavity, said needle assemblybeing removably attached to said tip of said barrel through engagementof said tip to said cavity so that said lumen is in fluid communicationwith said chamber.